The invention relates to a method, a device and a rivet for mechanical joining by means of punch riveting.
In known punch riveting with semitubular rivets the rivet is joined into the sheets to be joined by a linear movement without a prepunched hole.
In known clinching a punch penetrates during the working stroke in a linear motion into the sheets to be joined, whereby on the opposite side a solid or split die takes part in forming the die side of the clinch spot. A counterpunch is positioned in the split die.
The known wobbling movement is used for the forming of a rivet head on the face side by partial compressive deformation. The movement is also used in clinching, i.e. joining without using an auxiliary joining part (DE 198 43 874.5).
In known punch riveting heavy forces are required during the linear punch motion. Therefore the tool load is high which limits the application for high-strength sheet materials.
For the C-frames preferably used as tool frames, heavy forces limit the capacity and hence, the applicability of the method.
Due to the flow of the sheet material and the additional deformation of the rivet, a specific shape of the joint results during the linear punch movement.
It is the objective of this invention to reduce the forces occurring in punch riveting. This aims at extending the field of application of this process concerning high-strength materials and the accessibility of C-frames for large workpieces. It is another objective of this invention to eliminate the process""s weak points of drawing-in of the head and low expansion and thus to achieve a better appearance and higher joining strengths in equivalent joining jobs.
According to the invention, the problem is solved by a method in which, during the axial feeding motion of the semitubular rivet, the upper tool or/and a portion of the lower tool are given a wobbling additional movement in radial and/or tangential direction.
This additional movement is superimposed on the axial feeding motion during the whole or part of the joining process. The wobbling movement can take place tangentially, e.g. in a circular motion, radially outward from the centre, e.g. in a pivoting motion, and in a combined motion tangentially/radially, e.g. in a rosette kinematics. Due to the wobbling movement the material is partially deformed which distinctly reduces the process forces.
Advantageously, for punch riveting with semitubular rivet a rivet is used the head of which has a convex elevation and the shank inner and outer contours of which beginning from the rivet foot in direction to the rivet head are shaped such that a continuous increase of the cross-sectional area of the rivet shank occurs.
It is also advantageous, for a simultaneous active additional movement of the upper tool and a portion of the lower tool, to make these move synchronously such that the introduce axial forces of the punches oppose each other directly.
According to the invention, the problem is further solved by a device by means of which the upper tool or/and a portion of the lower tool as counterpunch can be moved in a wobbling manner in radial and/or tangential directions.
In the device according to the invention, at least one of the tools (upper tool, a portion of the lower tool as counterpunch) is displaced in a wobbling manner in addition to the known axial feed motion. Both opposing tools can execute this wobbling motion simultaneously and, in a special embodiment, synchronizedly.
Advantageously, the die of the device is a split die. This allows to realize a special material flow on the die side of the joint.
In an embodiment of the device with only the upper tool wobbling, the lower tool, which in other case is split, is designed as a one-piece part
According to the invention, the problem is further solved by a rivet whereby the shank inner and outer contours, beginning from the rivet foot in direction of the rivet head, are shaped such that a continuous increase of the cross-sectional area of the rivet shank results.
Advantageously, the rivet has a convex elevation at the rivet head.
It is also advantageous that the inner and outer profiles of the semitubular rivet are described by two tractrix curves in each case whereby the start points of the curves are situated in direction of the rivet foot and rivet head, respectively, and the transition of the curves is tangential in the centre.